1. Field of the Invention
The present invention relates to a transfer nip roller, a transfer device, and an image forming apparatus.
2. Description of the Related Art
A transfer nip roller described in Japanese Patent Application Laid-open Publication No. 2004-117674 has been known. The transfer nip roller comes in contact with the surface of an endlessly moving intermediate transfer belt to form a transfer nip. Through the effect of a transfer bias supplied from a power supply, a toner image on the intermediate transfer belt is transferred onto a recording sheet nipped in the transfer nip. The base material of the transfer nip roller is a core having a shaft supported rotatably on a bearing, and the peripheral surface of the roller portion of the core is covered with a covering portion having an elastic layer and a surface layer. In this configuration, contact pressure acting on the transfer nip deforms the elastic layer freely to enhance the adhesion between the transfer nip roller and the intermediate transfer belt and form a wider transfer nip. Moreover, using a material having a smaller friction coefficient, such as fluororesin, as the surface layer formed independent of the elastic layer suppresses the adhesion of toner to the surface of the transfer nip roller.
The transfer nip roller, however, has a problem that a crack is easily formed on the surface layer. Specifically, the surface made of a material showing a better parting property against toner is inferior in elasticity to the elastic layer, but is forced to deform pursuant to the elastic deformation of the elastic layer underneath the surface layer on the transfer nip. When the transfer nip roller passes through the transfer nip to allow the elastic layer to return to its original form, the surface layer, following the move of the elastic layer, also returns to its original form. If the surface layer repeats such forced deformation and recovery, a crack is eventually formed on the surface layer as a result of a fatigue failure at a local spot. The crack formed on the surface layer catches toner to accumulate it inside the crack, and accumulated toner is eventually transferred to a recording sheet to cause the back soiling of the recording sheet.
A conventional transfer nip roller sometimes causes such a crack in a short period from the initial state to the completion of printing of several ten thousands of sheets. The conventional transfer nip roller, therefore, needs to be replaced every time printing of several ten thousands of sheets is over. This results in cost burden and maintenance burden on a user. A demand for lower cost and better maintenance efficiency is high in these days, and such a demand leads to a call for the development of a transfer nip roller having a lifetime long enough to withstand workload of printing of about 300,000 sheets.